The present invention relates to a method for calibrating photomultiplier tubes in a scintillation camera having a plurality of light sources.
In the human body, increased metabolic activity is associated with an increase in emitted radiation. In the field of nuclear medicine, increased metabolic activity within a patient is detected using a radiation detector such as a scintillation camera.
Scintillation cameras are well known in the art, and are used for medical diagnostics. A patient ingests, or inhales or is injected with a small quantity of a radioactive isotope. The radioactive isotope emits photons that are detected by a scintillation medium in the scintillation camera. The scintillation medium is commonly a sodium iodide crystal, BGO or other. The scintillation medium emits a small flash or scintillation of light, in response to stimulating radiation, such as from a patient. The intensity of the scintillation of light is proportional to the energy of the stimulating photon, such as a gamma photon. Note that the relationship between the intensity of the scintillation of light and the gamma photon is not linear.
A conventional scintillation camera such as a gamma camera includes a detector which converts into electrical signals gamma rays emitted from a patient after radioisotope has been administered to the patient. The detector includes a scintillator and photomultiplier tubes. The gamma rays are directed to the scintillator which absorbs the radiation and produces, in response, a very small flash of light. An array of photodetectors, which are placed in optical communication with the scintillation crystal, converts these flashes into electrical signals which are subsequently processed. The processing enables the camera to produce an image of the distribution of the radioisotope within the patient.
Gamma radiation is emitted in all directions and it is necessary to collimate the radiation before the radiation impinges on the crystal scintillator. This is accomplished by a collimator which is a sheet of absorbing material, usually lead, perforated by relatively narrow channels. The collimator is detachably secured to the detector head, allowing the collimator to be changed to enable the detector head to be used with the different energies of isotope to suit particular characteristics of the patient study. A collimator may vary considerably in weight to match the isotope or study type.
Scintillation cameras are used to take four basic types of pictures: spot views, whole body views, partial whole body views, SPECT views, and whole body SPECT views.
A spot view is an image of a part of a patient. The area of the spot view is less than or equal to the size of the field of view of the gamma camera. In order to be able to achieve a full range of spot views, a gamma camera must be positionable at any location relative to a patient.
One type of whole body view is a series of spot views fitted together such that the whole body of the patient may be viewed at one time. Another type of whole body view is a continuous scan of the whole body of the patient. A partial whole body view is simply a whole body view that covers only part of the body of the patient. In order to be able to achieve a whole body view, a gamma camera must be positionable at any location relative to a patient in an automated sequence of views.
The acronym xe2x80x9cSPECTxe2x80x9d stands for single photon emission computerized tomography. A SPECT view is a series of slice-like images of the patient. The slice-like images are often, but not necessarily, transversely oriented with respect to the patient. Each slice-like image is made up of multiple views taken at different angles around the patient, the data from the various views being combined to form the slice-like image. In order to be able to achieve a SPECT view, a scintillation camera must be rotatable around a patient, with the direction of the detector head of the scintillation camera pointing in a series of known and precise directions such that reprojection of the data can be accurately undertaken.
A whole body SPECT view is a series of parallel slice-like transverse images of a patient. Typically, a whole body SPECT view consists of sixty four spaced apart SPECT views. A whole body SPECT view results from the simultaneous generation of whole body and SPECT image data. In order to be able to achieve a whole body SPECT view, a scintillation camera must be rotatable around a patient, with the direction of the detector head of the scintillation camera pointing in a series of known and precise directions such that reprojection of the data can be accurately undertaken.
Therefore, in order that the radiation detector be capable of achieving the above four basic views, the support structure for the radiation detector must be capable of positioning the radiation detector in any position relative to the patient. Furthermore, the support structure must be capable of moving the radiation detector relative to the patient in a controlled manner along any path.
In order to operate a scintillation camera as described above, the patient should be supported horizontally on a patient support or stretcher.
The detector head of the scintillation camera must be able to pass underneath the patient. Therefore, in order for the scintillation camera to generate images from underneath the patient, the patient support must be thin. However, detector heads are generally supported by a pair of arms which extend from a gantry. Thus, the patient support generally must be cantilevered in order for the detector head to be able to pass underneath the patient without contacting any supporting structure associated with the patient support. The design of a cantilevered patient support that is thin enough to work properly with a scintillation camera is exceedingly difficult. Expensive materials and materials that are difficult to work with, such as carbon fibre, are often used in the design of such cantilevered patient supports.
A certain design of gantry or support structure for a scintillation camera includes a frame upon which a vertically oriented annular support rotates. Extending out from the rotating support is an elongate support. The elongate generally comprises a pair of arms. The pair of arms generally extends through a corresponding pair of apertures in the rotating support. One end of the pair of arms supports the detector head on one side of the annular support. The other end of the pair of arms supports a counter balance weight. Thus, the elongate support is counterbalanced with a counterweight on the opposite side of the detector head.
With such a design of support structure for a scintillation camera, a patient must lie on a horizontally oriented patient support. The patient support must be cantilevered so that the detector head can pass underneath the patient. If the detector head must pass underneath only one end of the patient, such as the patient""s head, the cantilevered portion of the patient support is not long enough to cause serious difficulties in the design of the cantilevered patient support. However, if the camera must be able to pass under the entire length of the patient, the entire patient must be supported by the cantilevered portion of the patient support. As the cantilevered portion of the patient support must be thin so as not to interfere with the generation of images by the scintillation camera, serious design difficulties are encountered.
Among the advantages associated with such a design of support structure is that a patient may be partially passed through the orifice defined by the annular support so that the pair of arms need not be as long. However, the patient support must be able to support the patient in this position relative to the annular support, must be accurately positionable relative to the annular support, and must not interfere either with the rotation of the annular support or with the cable which will inevitably extend from the detector head to a nearby computer or other user control.
The photomultiplier tubes in a scintillation camera generate electric signals. The signals are processed, and images are created corresponding to the radiation emitted by the patient.
The photomultiplier tubes in a scintillation camera must be calibrated from time to time, that is, gain calibration must be performed, to ensure that their output remains constant.
Scintillation cameras generally include an light emitting diode for each photomultiplier tube. Typically, to calibrate the photomultiplier tube, the light emitting diode for that particular photomultiplier tube is pulsed, that is, is activated so as to provide a pulse of light. The output of the photomultiplier tube is compared with a known or expected value, such as a previously measured output of the photomultiplier tube.
If the output of the photomultiplier tube corresponds to the expected value, within a certain tolerance, the photomultiplier tube likely needs no calibration.
If the output of the photomultiplier tube is different from the expected value, that is, outside the tolerance, the photomultiplier tube is probably in need of calibration. However, it is possible that the photomultiplier tube does not need calibration, but rather that it is the output of the light emitting diode that has changed.
A prior art method of determining whether the output of the light emitting diode has changed is disclosed in U.S. Pat. No. 5,237,173 to Stark et al. In the disclosed method, the light emitting diode is pulsed. The output of the corresponding photomultiplier tube is then compared with the expected value. The output of the surrounding photomultiplier tubes are then compared with their expected values. If the outputs of all the photomultiplier tubes do not equal their expected values, within an appropriate tolerance, then it is likely that it is the light emitting diode that is malfunctioning. However, if it is only the output of the photomultiplier tube being calibrated that does not correspond to its expected value, then it is likely that it is the photomultiplier tube that is in need of calibration.
The above process is repeated for each photomultiplier tube. Accordingly, one disadvantage of this prior art method is that it is slow.
An object of the invention is to provide an improved an improved method for calibrating photomultiplier tubes in a scintillation camera.
The invention relates to a method and apparatus for calibrating photomultiplier tubes in a scintillation camera having a plurality of light sources. The method includes the steps of: pulsing all light sources simultaneously; reading the output of each photomultiplier tube; comparing the output of each photomultiplier tube with an expected value; determining whether the output of each photomultiplier tube is within a first specified tolerance; and adjusting each photomultiplier tube if the output of the photomultiplier tube is not within the first specified tolerance.
An embodiment of the invention also relates to a method and apparatus for calibrating photomultiplier tubes in a scintillation camera having a plurality of light sources. The method includes the steps of: pulsing all light sources simultaneously; reading the output of each photomultiplier tube; summing the outputs of the photomultiplier tubes; comparing the sum of the outputs of the photomultiplier tubes with an expected sum; determining whether the sum of the outputs of the photomultiplier tubes is within a second specified tolerance; adjusting the light sources if necessary, if the sum of the outputs of the photomultiplier tubes is not within the second specified tolerance, and repeating the above steps; comparing the output of each photomultiplier tube with an expected value; determining whether the output of each photomultiplier tube is within a first specified tolerance; adjusting each photomultiplier tube if the output of the photomultiplier tube is not within the first specified tolerance; comparing the output of each photomultiplier tube with the sum of the outputs of the photomultiplier tubes divided by the number of photomultiplier tubes; determining whether the output of each photomultiplier tube is within a third specified tolerance; and adjusting each photomultiplier tube if the photomultiplier tube is not within the third specified tolerance.
The invention also relates to a scintillation camera for obtaining a distribution image of incident gamma rays from a subject, the camera having a scintillator for emitting flashes of light due to incident gamma rays, a plurality of photomultiplier tubes optically coupled with said scintillator for converting the light flashes into respective electric signals which are individually detectable. The scintillation camera includes: a plurality of pulsible light sources, the plurality of light sources being simultaneously pulsible; pulsing means for pulsing the light sources simultaneously; and gain calibration means for comparing an output of each photomultiplier tube to an expected value, and carrying out an effective gain adjustment for each photomultiplier tube.
Other advantages, objects and features of the present invention will be readily apparent to those skilled in the art from a review of the following detailed description of preferred embodiments in conjunction with the accompanying drawings and claims.